Abstract

Glycine phases formed when aqueous solutions were frozen and subsequently heated under different conditions were studied by Raman scattering,x-ray diffraction, and differential scanning calorimetry(DSC) techniques. Crystallization of ice Ih was observed in all the cases. On cooling at the rates of 0.5 K/min and 5 K/min, glassy glycine was formed as an intermediate phase which lived about 1 min or less only, and then transformed into β-polymorph of glycine. Quench cooling of glycine solutions (15% w/w) in liquid nitrogen resulted in the formation of a mixture of crystalline water ice Ih and a glassy glycine, which could be preserved at cryogenic temperatures (80 K) for an indefinitely long time. This mixture remained also quite stable for some time after heating above the cryogenic temperature. Subsequent heating under various conditions resulted in the transformation of the glycine glass into an unknown crystalline phase (glycine “X-phase”) at 209–216 K, which at 218–226 K transformed into β-polymorph of glycine. The “X-phase” was characterized by Raman spectroscopy; it could be obtained in noticeable amounts using a special preparation technique and tentatively characterized by x-ray powder diffraction (P2, a = 6.648 Å, b = 25.867 Å, c = 5.610 Å, β = 113.12º); the formation of “X-phase” from the glycine glassy phase and its transformation into β-polymorph were followed by DSC.Raman scattering technique with its power for unambiguous identification of the crystalline and glassypolymorphs without limitation on the crystallite size helped us to follow the phase transformations during quenching, heating, and annealing. The experimental findings are considered in relation to the problem of control of glycine polymorphism on crystallization.

Received 18 May 2012Accepted 13 July 2012Published online 10 August 2012

Acknowledgments:

This work was supported by the Russian Federation for Basic Research (RFBR) (Grant Nos. 12-03-00424 and 11-03-12114-ofi-m-2011), an Integration Project of SB RAS No. 108, and Russian Ministry of Education and Science (State Contract of RF No. 14.740.11.1023). The work was carried out using the equipment belonging to the SSTRC and within the framework of State Contract No. 16.552.11.7044. The authors thank A. F. Achkasov for providing pure samples of α- and γ- glycine polymorphs.